Apparatus and method for determining coefficient of friction

ABSTRACT

An apparatus for obtaining data equatable to the static and dynamic coefficients of friction of the working surface of a friction disc comprises fixture means for mounting and holding the disc for limited pivotal movement about an axes disposed parallel to the central, longitudinal axis of the disc, load application means for urging the working surface of the disc into engagement with a rotating surface to induce pivoting of the fixture and disc, and means for indicating and ascertaining the torque applied to the disc by the rotating surface, including a strain-gaged torque link and associated recording equipment.

United States Patent [1 1 Kronenberg et al.

[ 51 Feb. 20, 1973 [54] APPARATUS AND METHOD FOR DETERMINING COEFFICIENTOF FRICTION [75] Inventors: Francis E. Kronenberg; Alexius B. PrimaryExaminer Louis Prince Olson bothGof i; James 12 Assistant ExaminerDanielM. Yasich Morton; era d Rohw er, Peoria; Hamid w. winner East AttorneyFryer, T ensvold, Feix, Phlll|ps& Lemplo Peoria, all of 111. I 57]ABSTRACT [73] Asslgneez (fatal-plum Traqor Peona An apparatus forobtaining data equatable to the static [22] Filed: March 1, 1971 anddynamic coefficients of friction of the working 21 A L N 122 598 surfaceof a friction disc comprises fixture means for 1 pp 0 mounting andholding the disc for limited pivotal movement about an axes disposedparallel to the cen- UsS. 73/9, tra] longitudinal axis of the disc loadapplication- [51] Int. Cl. ..G01n 19/02 means f r urging the workingSurface f the disc i [58] Field Of Search ..73/9, engagement with arotating surface to induce pivoting of the fixture and disc, and meansfor indicating and [56] References Cited ascertaining the torque appliedto the disc by the UNITED STATES ATENTS rotating surface, includingastrain-gaged torque link.

and associated recording equipment. 1,490,603 4/1924 Elverson ..73/93,230,762 1/1966 Doran ..73/9 X 12 Claims, 5 Drawing Figures 48 7 3 I 15OSCILLOGRAPH A IR m. SIGNAL 2 OSCILLOGRArn COND'TIONER H i 47 SOURCE a'4 l l k 7 L g 40 co l af'r l rkzla I b CONTROL CONTROL ELECTRONIC 42 lSYSTEM COUNTER VALVE g 50 Y 33 :r 44 Z63 27 34 29 I6 X 22 POWER SUPPLY 2I '5 I g 32 2 37 I8 .6 35 4 m PATENTEDFEB201975 3.717.025

sum 10F 2 E E L I 39 SOURCE ATTORNE S PATENTED B20190 sum 2 or 2 STATICCOEFFICIENT AVERAGE BREAKAWAY TORQUE= |9.6 FT."LB.

TIME. (I LINE PER sEc.)

:EJLE S TIME (I LINE PER SEC.)

BY a7wjw 9,

DYNAMIC COEFFICIENT STABILIZED TORQUE TRACE 9.3 FT.-LB.

20 l: E 5:; n:

20 SECONDS 2000 E 22 I000 Q LL] 5 O m INVENTORS FRANCI E. KRONENBERGALEXIUi1 B. OLSON JAMES POIROT GERALD D. ROHWEDER HAROLD W. WINKLER ATTRNEYS APPARATUS AND METHOD FOR DETERMINING COEFFICIENT OF FRICTIONBACKGROUND OF THE INVENTION Commercial acceptance of large frictiondiscs, such as those used in clutches and brakes, dictates the need foran economical and rapid method for accurately determining the static anddynamic coefficients of friction thereof. The automotive industry hasdeveloped test machines which are normally limited to the handling offriction discs having a'maximum diameter approximating 6 inches. Thediscs are generally mounted on the driven shaft of a variable speedmotor and are subjected to axial loading for test purposes. Largefrictiondiscs, such as those used in heavy earthmoving machinery, arenormally tested by cutting the friction material of a disc into segmentsand by mounting the segments onto a smaller disc, thus rendering thetested structure unusable for the intended, commercial applications.

An attempt to utilize the automotive-type testing methods for thetesting of large discs would give rise to insurmountable problems. Forexample, the spinning of a large disc, having an outside diameter of 30inches, would be highly dangerous to the machines operator due to largemass involved. In addition, a highly rugged and specially designedmachine would be required to withstand the very high loading andhorsepower requirements encountered therewith. Such a machine would beunduly complex, expensive to manufacture, and difficult to set-up andoperate. Also, the machine would be incapable of testing a sufficientnumber of test samples per unit time to satisfyaccepted quality controlstandards.

SUMMARY AND OBJECTS OF THE INVENTION This invention relates to afriction testing apparatus and method for overcoming the. above,brieflydescribed problems. Thc apparatus essentially comprises means forpivotally mounting a test sample thereon and means for rotating asurface against a working surface of the test sample to induce pivotingof the sample. Such pivoting action can be mechanically transposed intoa working torque utilized to compute the coefficient of friction of thetested working surface, along with related design parameters. As will behereinafter more fully understood, the type of groove pattern formed onthe tested surface and the diameter of the tested sample or disc havelittle, if any, effect on the ability to accurately compute thecoefficient of friction thereof.

In the preferredembodiment of this invention, the rotated surface whichengages the test sample is formed on the free end of a mandrel and loadapplication means are employed to compress the mandrel and working testsurface together under a selectively variable, axial load. Torquedeveloped inresponse to pivoting of the test sample is preferablymeasured by a strain-gaged torque link and read'out on suitablyintegrated recording equipment.

In accordance with the above discussions, an object of this invention isto provide a friction testing apparatus and method which satisfyaccepted quality assurance programs by economically, expeditiously andaccurately obtaining the coefficients of friction of a tested surface,such as the working surface of friction BRIEF DESCRIPTION OF THEDRAWINGS FIG. I is a partially schematic, longitudinal section of anapparatus embodying this invention;

FIG. 2 is a transverse section taken in the direction of arrows IIII inFIG. I;

FIG. 3 is a fragmentary view of disc mounted in th FIG. I apparatus;

FIG. 4 shows graphical torque traces equatable'to the static and dynamiccoefficients of friction of a tested sample;

FIG. 5 shows a trace of spindle speeds corresponding to the traces shownin FIG. 4.

I DESCRIPTION OF THE PREFERRED EMBODIMENT The FIG/l apparatus comprisesa stationary base and integrated support stand adapted to have a testsample, such as an axially engageable friction clutch or brake disc 11,detachably mounted thereon by fixture means 12. The fixture meanscomprises an interchangeable and vertically adjustable (not shown)clamping means 13 releasably attached to an upstanding member 14. Twopins 15 and 16 and a lower ledge 17, secured to a vertically disposedmember 14,

cooperate with the clamping means to fixedly mount the disc on member14.

The pins engage internally formed spline teeth of the standard disc(FIG. 3) to prevent it from rotating relative to the fixture meansjThepins could be otherwise disposed on member 14 to engage a friction dischaving teeth formed externally thereon, for example. The pins aresecured to member 14 with upper pin 15 having its free end removablymounted in an aperture formed in clamp 13. I

The lower end of the member is secured to a first end of a horizontallydisposed shaft 18 rotatably supported in bearing means 19 which furtherpermits the shaft to move axially in the direction of its longitudinalaxis X. The shaft is adapted to impose a selectively variable axialloading force on the disc by load application means, shown in the formof a diaphragm-type rotochamber air cylinder 20. A conventional aircontrol valve 21 is arranged to selectively communicate a pressurizedair source to the actuating chamber of cylinder 20. Leftward movement ofa rod 22 is transmitted to the second end of shaft 18 through a thrustbearing arrangement 23. The thrust bearing permits shaft 18 to rotateand minimizes errors in the hereinafter described measured torque byisolating torsional loads which would otherwise be imparted thereto bythe air cylinder.

A rotary means or mandrel 24 is driven by a power output shaft 25 of amanually controlled power supply means 26, such as anelectric motor.Shaft 25 is coupled to the mandrel by'a self-aligning coupling 27, aspindle 28, and a universal joint 29. The mandrel is preferably -in theform of a hollow cylinder, having closely controlled inside and outsidediameters, to

- closely simulate average sliding velocities encountered by the disc incommercial clutch and brake applications.

The last-mentioned drive train elements are normally disposed forrotation about a common longitudinal axis Y, normally coincident withaxis X, and accommodate a surface 30 of the mandrel in its assimilationof an actual clutch application. For example, the universal jointassures correct alignment as between surface 30 and a juxtaposed,engageable working surface 31 of the fric-. 1 tion disc 11. Mandrelsurface 30 is preferably hardened and ground to a fine micro-finish.

Spindle 28 is supported for rotation by spaced bearings 32 and 33,separated by a cylindrical spacer 34. The spacer aids in counteractingaxial loads, imposed on mandrel 24 by air cylinder 20, by its engagementwith annular bearing 32 which in turn abuts a seal retainer 35 securedto stationary support and housing 10. When oil is used to assimilatecommercial clutch lubrication and cooling conditions, the cavitiesformed in housing 10 provide a sump which is sealed by means of annularseals 36 and 37.

In particular, the apparatus is constructed to permit the disc to rundry or with the mandrel and a portion of the disc completely submergedbeneath an oil level 38 toassimilate an actual clutch application.The-oil may be pumped across the surface of disc 11, if so desired, andthe oil temperature in the sump can be controlled by a suitablyintegrated heat exchanger (not shown). A detachable splash guard 39,attached to above housing and support 10, is normally employed to closethe system to prevent the ingress of contaminants therein.

In general, the FIG. 1 apparatus is adapted for accurately andexpeditiously determining the static and dynamic coefficients offriction of disc surface 31 by equating same to a torque read-outdeveloped in response to a pivoting of disc 11 and attached member 14.In particular, rotation of mandrel surface 30 against the abutting andcompressed working surface of the disc will induce pivoting of the discand member to flex a hereinafter described strain-gaged torque link 40(FIG. 2)..The surfaces abut under a predetermined unit pressure asgoverned by selectively controlled air-actuated cylinder 20.

As shown in FIG. 2, the test sample and thus member 14 are induced topivot clockwise about axis X. The torque'imparted to the test sample canbe equated to the coefficient of friction of tested surface 31 whereasthe unit pressure applied thereto and the rotary speed of the mandrelare dictated by predetermined test parameters. The apparatus affords theoperator precise control of the resulting unit pressure and slidingvelocity to provide accurate data for measuring the torque imparted tothe disc. For example, mandrel 28 can rotate at 3,000 rpm and a load of630 pounds can be simultaneously applied to the test sample. The primarypurpose of the cylindrically shaped mandrel is to minimize surfacevelocity gradient as between the inside and outside diameters thereof.

The torque transmitted by mandrel surface 30 to tested surface 31 ismeasured by a straimgaged, flexible torque link 40 of the torqueindicator means. The cantilevered link is fixedly attached at one end toa cupshaped member 41, secured to an end of shaft 18. The

link bears against a collar bearing 42 upon pivoting of support 14 (FIG.2) to bend the reed-type link to its dotted-line position. The collarbearing, rotatably mounted on a bolt 43 secured to frame 10, reducespotential errors which might otherwise arise due to the frictional dragoccasioned between the link and the collar bolt.

A standard strain gage 44 is preferably attached to the link at itspoint of maximum flexure (FIG. 2) to maximize the electrical outputsignal. The mean diameter 45 of the friction material on disc 11 ispreferably intersected by horizontally disposed pivotal axis X and thelink is normally arranged in a perpendicularly disposed plane whichintersects the pivot axis. A crosshatched area 46 is schematicallyillustrated in FIG. 2 to indicate the area of surface contact betweenthe disc and mandrel.

The torque imparted to disc and measured by the strain gage may bereceived by a standard signal conditioner 47, manufactured by BrushElectronic Co. and identified as Type No. BL-SZO. The output of theinmitted to an oscillograph 48, also manufactured by such company andidentified as Type No. BL-202, to provide visual torque read-out data.Spindle 28has a toothed spline 49 formed thereon to provide means formagnetically detecting the rotational speed of the spindle by a magneticpick-up 50. Such speed is then displayed on a second oscillograph 51 viaan electronic counter 52 and signal conditioner 53 also of conventionaldesign.

FIGS. 4 and 5 graphically illustrate a typical test run wherein theplotted torque and speed traces, obtained from oscillographs 48 and 51,are displayed for use in computing the static and dynamiccoefficients'of friction of disc surface 31. The general formula usedfor calculating the coefficients of friction is:

where: p. coefficient of friction t= the torque applied to link 40(ft.-lb.)

F the normal force applied to disc 11 by cylinder 20 and shaft 18 (1b.)

R,,, the'mean radius of mandrel 24 (in.)

A typical test sequence comprised mounting disc 11 on support member 14by means of pins 15 and 16 and clamp 13. The mean radius-of mandrel 24was measured at 2.0 in. Control valve 21 was actuated to pressurize theactuating chamber of cylinder 20 at 30 psi to apply an axial load of626.6 lb. to shaft 18 and thus disc surface 31. I

The hand lever of the control system for power supply means 26 was movedgradually until the motor was energized sufficiently to rotate mandrel24 to the motor was allowed to run constantly at 2,000 rpm. The dynamicand stabilized torque reading at 2,000

tegrated bridge connection (not shown) may be trans- After completion ofthe breakaway test procedure,

rpm (9.3 ft.-lb.) was then taken seconds after breakaway (FIG. 4). Whena dynamic coefficient of friction readout is desired at a lower slidingvelocity, the motor speed can be reduced to achieve same.

Applying the above formula to the aforementioned test results, whereinthe average breakaway torque and stabilized dynamic torque equalled 19.6ft.-lb. and 9.3 ft.-lb. respectively, the following static (p anddynamic (p coefficients of friction were obtained:

What is claimed is:

1. An apparatus for obtaining the coefficient of friction of the surfaceof a test sample comprising fixture means fixedly mounting a test samplethereon, said fixture means mounted for limited pivotal movement about apivot axis in said apparatus,

rotating means having a surface positioned adjacent to said fixturemeans and rotatably engaging a surface of the test sample to inducepivoting of said fixture means about said pivot axis, load applicationmeans for pressing the surface of said rotating means and the surface ofthe test sample together under a predetermined force, and

torque indicator means responsive to pivotal movement of said fixturemeans for indicating the torque applied to the test sample by saidrotating means.

2. The invention of claim 1 wherein said fixture means comprises avertically disposed member, means for releasably detaching a test sampleon said member and a horizontally disposed shaft having said memberattached to a first end thereof.

3. The invention of claim 2 wherein said shaft is mounted for axialmovement along and rotational movement about said axis and wherein saidload application means is op'eratively connected to a second end of saidshaft.

4. The invention of claim 2 wherein said torque indicator meanscomprises a flexible link attached at one end thereof to said shaft forrotation therewith.

5. The invention of claim 4 wherein said torque indicator means furthercomprises means fixedly mounted in said apparatus, adjacent to saidlink, for engaging and bending said link in response to pivoting of saidmember and strain gage means attached to said link adjacent to its pointof maximum flexure.

6. The invention of claim 5 wherein said torque indicator means furthercomprises means for visually reading-out the torque applied to saidlink.

7. The invention of claim 1 wherein said rotating means comprises amandrel having a surface positioned closely adjacent to said fixturemeans to engage the surface of a test sample.

8. The invention of claim 7 wherein said mandrel is a hollow cylinder. 1

9. The invention of claim 7 wherein said mandrel is attached to aspindle rotatably mounted in said apparatus and power means forselectively rotating said spindle and said mandrel.

10. The invention of claim 9 further comprising means operativelyassociated with said mandrel for visuallydetectin the rotational speedthereof.

11. he inven ion of claim 1 wherein a test sample,

constituting an annular friction disc, is mounted on said fixture meansto have a tested surface thereof intersected by said pivot axis.

12. In an apparatus for obtaining the coefficient of friction of thesurface of a test sample, the invention comprising means fixedlymounting a test sample on fixture means in said apparatus, said fixturemeans mounted for limited pivotal movement about a pivot axis, means forrotating the surface of a mandrel against only a portion of the surfaceof said test sample to pivot said fixture means and test sample aboutsaid pivot axis, means for pressing said surfaces together under apredetermined load and means for measuring the torque applied to saidtest sample in response to pivoting of said fixture means and testsample.-

1. An apparatus for obtaining the coefficient of friction of the surfaceof a test sample comprising fixture means fixedly mounting a test samplethereon, said fixture means mounted for limited pivotal movement about apivot axis in said apparatus, rotating means having a surface positionedadjacent to said fixture means and rotatably engaging a surface of thetest sample to induce pivoting of said fixture means about said pivotaxis, load application means for pressing the surface of said rotatingmeans and the surface of the test sample together under a predeterminedforce, and torque indicator means responsive to pivotal movement of saidfixture means for indicating the torque applied to the test sample bysaid rotating means.
 1. An apparatus for obtaining the coefficient offriction of the surface of a test sample comprising fixture meansfixedly mounting a test sample thereon, said fixture means mounted forlimited pivotal movement about a pivot axis in said apparatus, rotatingmeans having a surface positioned adjacent to said fixture means androtatably engaging a surface of the test sample to induce pivoting ofsaid fixture means about said pivot axis, load application means forpressing the surface of said rotating means and the surface of the testsample together under a predetermined force, and torque indicator meansresponsive to pivotal movement of said fixture means for indicating thetorque applied to the test sample by said rotating means.
 2. Theinvention of claim 1 wherein said fixture means comprises a verticallydisposed member, means for releasably detaching a test sample on saidmember and a horizontally disposed shaft having said member attached toa first end thereof.
 3. The invention of claim 2 wherein said shaft ismounted for axial movement along and rotatioNal movement about said axisand wherein said load application means is operatively connected to asecond end of said shaft.
 4. The invention of claim 2 wherein saidtorque indicator means comprises a flexible link attached at one endthereof to said shaft for rotation therewith.
 5. The invention of claim4 wherein said torque indicator means further comprises means fixedlymounted in said apparatus, adjacent to said link, for engaging andbending said link in response to pivoting of said member and strain gagemeans attached to said link adjacent to its point of maximum flexure. 6.The invention of claim 5 wherein said torque indicator means furthercomprises means for visually reading-out the torque applied to saidlink.
 7. The invention of claim 1 wherein said rotating means comprisesa mandrel having a surface positioned closely adjacent to said fixturemeans to engage the surface of a test sample.
 8. The invention of claim7 wherein said mandrel is a hollow cylinder.
 9. The invention of claim 7wherein said mandrel is attached to a spindle rotatably mounted in saidapparatus and power means for selectively rotating said spindle and saidmandrel.
 10. The invention of claim 9 further comprising meansoperatively associated with said mandrel for visually detecting therotational speed thereof.
 11. The invention of claim 1 wherein a testsample, constituting an annular friction disc, is mounted on saidfixture means to have a tested surface thereof intersected by said pivotaxis.